A solder alloy is used when mounting electronic parts on a printed circuit board in electronic equipment. Solder alloys having various compositions are used taking into consideration thermal effects on electronic parts and printed circuit boards, workability, and reliability after connection.
A representative example of a soldering method used with these solder alloys is the flow soldering method using a molten solder bath.
The flow soldering method is a method in which after a flux is applied over the entire surface of one side of a printed circuit board on which electronic parts are mounted, preheating is carried out, and one side of the printed circuit board is made to contact a molten solder bath. In recent years, the wave soldering method has become typical as a mounting method for printed circuit boards in mass production where costs are important. This is a method in which molten solder is made to flow from a nozzle to form a stable wave of molten solder, and a printed circuit board is contacted with the top of the wave.
During soldering operations, various soldering defects such as unsoldered portions, bridges, and voids may occur. Particularly with the flow soldering method, since solder is always flowing, the solder which is discharged from the spouting nozzle falls into a stationary area within the bath in the vicinity of the nozzle. Turbulence is produced in the area where the flow of solder falls off, and oxides referred to as dross form due to entrainment of oxygen. If the dross accumulates in the vicinity of the nozzle, the stability of the molten solder wave is disturbed, a portion of the dross which is entrained in the solder wave adheres to the printed circuit board, and this may lead to soldering defects, so it is necessary to periodically remove dross which accumulates on the surface of the solder bath.
A similar problem with the formation of dross can be seen even when using a stationary solder bath when carrying out soldering at a high temperature in an oxidizing atmosphere, such as when carrying out soldering of urethane coated copper wire.
If a large amount of such dross is produced, the time required for dross removal and the amount of discarded solder increase, so running costs increase. Accordingly, it is desirable to decrease the amount of dross from the standpoints of maintenance of a solder bath and cost.
With lead free solder, the material costs are higher, so there is all the greater desire for a decrease in the amount of dross.
In order to reduce dross, as a way of solving the problem by means of the solder alloy, a deoxidizing alloy to which is added an element having an oxidation suppressing effect has been used. A representative example of such an oxidation suppressing element is P. By preferentially reacting with oxygen, P has the effect of suppressing the oxidation of Sn or Pb, which are principal components of a solder alloy.
The oxidation suppressing effect of P is so-called sacrificial oxidation, and as a result it leads to a suppression of dross. P is selectively consumed and its concentration in dross increases, and it is discharged to the exterior of a soldering tank together with dross. Therefore, the amount of P, i.e., of an oxidation suppressing element in the solder alloy decreases, and eventually it is completely consumed.
If an oxidation suppressing element in a solder alloy is entirely consumed, the dross suppressing effect disappears, so the amount of dross which is produced increases, and not only does the amount of discarded solder increase, but soldering defects caused by dross occur, the overall defect rate in flow soldering increases, and running costs may increase.
In this manner, in the prior art, it was known to add P to a solder bath. However, supply of P to a solder bath used a deoxidizing alloy having a much higher concentration of P (such as 0.05-3% P) than the prescribed concentration of P in a solder alloy in a solder bath, so even if a small amount of a deoxidizing alloy was added, a large fluctuation in the P content in the plating bath at the time of replenishing was unavoidable (see Japanese Published Unexamined Patent Application Sho 54-84817).
In addition, there are also examples of solder alloys containing at most 50 ppm of P (see Japanese Published Unexamined Patent Application Sho 55-75893) or containing 0.1-1% of P (see Japanese Published Unexamined Patent Application Hei 11-333589), but these were not used for replenishing a solder bath.
With regards to the oxidation suppressing element P, in the past, the P concentration was focused on, and when the P concentration of the solder in the solder bath decreased, the P concentration of solder in a solder bath was restored by charging a small amount of a solder alloy having a high P concentration.
In a soldering tank, soldering of a large number of printed circuit boards is carried out, so the solder alloy in the solder bath adheres to the printed circuit board and decreases in amount. In the past, supply of the amount by which the solder decreased was carried out using a solder alloy not containing P. Namely, the decrease in the P concentration and the decrease in the solder alloy were separately controlled. As a result, a small amount of a solder alloy having a high concentration of P was charged into the solder bath as the P in the solder in the solder bath decreased, and a solder alloy not containing any P was supplied as the solder in the solder bath decreased.
In the above-described case, when the solder alloy forming the solder bath already contained P, it was thought that the same solder alloy could be used as a solder alloy for replenishing. This corresponds to supplying a solder alloy having the same P content as the solder alloy already filling the soldering tank. However, in the case of wave soldering, there is large consumption of P in the solder bath. If a solder alloy having the same concentration of P as that of the solder bath was supplied, an amount of P corresponding to the amount of consumed P could not be replenished.
Accordingly, in this case, too, as the P in the solder in the solder bath decreased, a small amount of a deoxidizing alloy having a high concentration of P was charged so as to restore the P concentration in the solder bath.
In fact, in the past as well, when an oxidation suppressing element in a solder bath was consumed, a method of coping was employed in which an amount corresponding to the consumed oxidation suppressing element was supplied, as stated earlier. For example, a mother alloy adjusted to have a high concentration of an oxidation suppressing element was supplied to a solder bath periodically, such as every day or 2-4 times per month, and the concentration of an oxidation suppressing element in the bath was adjusted.
However, each time this adjusting operation was carried out, it was necessary to perform troublesome operations such as weighing the amount of mother alloy to be charged, charging into the tank and stirring for a fixed length of time, and checking the concentration in the tank after adjustment.